A combination of expensive health insurance and an ever-increasing rate of obesity appear to be behind a startling fall by the US in the world rankings of life expectancy.

Despite being one of the richest countries in the world, America has dropped from 11th to 42nd place in 20 years, according to official US figures.

Dr Christopher Murray, head of the Institute of Health Metrics and Evaluation at the University of Washington, said: "Something's wrong here when one of the richest countries in the world, the one that spends the most on health care, is not able to keep up with other countries."

The lack of health care available to many Americans - 45 million have no health insurance - is set to be one of the biggest issues in next year's presidential election campaign. The Democratic contenders all promise universal health care.

The decline reflects the disparity in wealth.The life expectancy of African Americans is 73.3 compared with 77.9 for whites. For African-American males, it is even shorter: 69.8.

Jim McDermott, a Democratic Congressman, said: "Health care coverage is the single biggest domestic crisis facing America. It threatens all but the wealthiest Americans. "If you aren't part of the richest 1%, then you know you are living one phone call, accident or illness away from financial ruin because of a medical crisis."

Obesity is frequently cited as among the causes of lower life expectancy. Almost a third of US adults are obese, according to the National Centre for Health Statistics, which compared US life expectancy with the rest of the world.

Paul Terry, an assistant professor of epidemiology at Emory University in Atlanta, said: "The US has the resources that allow people to get fat and lazy."

The drop is also due to improved health care, nutrition and lifestyle elsewhere in the world. Countries with longer life expectancy include most of Europe, Japan, Singapore and Jordan.

The US also has a higher infant mortality rate than many other countries: 6.8 deaths for every 1,000 live births. The worst life expectancy figures are in Africa, with Swaziland at the bottom, at 34.1 years.